1. Field of Invention
This invention relates to an ultraviolet fluorescent lamp and, more particularly, to an ultraviolet fluorescent lamp used for accelerated artificial exposure test on polymer material.
2. Description of the Related Art
For the testing of the weatherability of polymer materials such as paint films and synthetic resins, an accelerated artificial exposure apparatus is used. Such apparatus ideally uses a light source, which has a spectral distribution similar to that of sunlight in the entire spectral range. However, it is very difficult to realize such a light source. Xenon lamps have the most similar spectral distribution to that of sunlight among presently available light sources. The xenon lamp, however, is held at very high temperatures during its operation. Therefore, when it is operated, i.e., held "on", for a long time, solarization of the glass constituting its bulb occurs, resulting in early reduction of short wavelength ultraviolet rays in the neighborhood of 300 nm.
Ultraviolet fluorescent lamps are also used as the light source of accelerated artificial exposure apparatus. With these lamps, the solarization of glass does not occur owing to low lamp temperature. In addition, changes of the spectral distribution with time are not substantially influenced by the wavelength. With the ultraviolet fluorescent lamp, however, the irradiance is subject to the influence of the lamp temperature. Therefore, it is necessary to control the ambient temperature independently of the irradiated material.
An erythemal or sun lamp (FL20S.multidot.E), which is one kind of ultraviolet fluorescent lamp, has a spectral irradiance characteristic as shown by curve A in FIG. 1. Its spectral irradiance in a wavelength range of 270 to 295 nm is considerably high compared to that of sunlight as shown by curve B. With sunlight, the irradiance is reduced sharply as the wavelength becomes less than 300 nm. More specifically, the irradiance is reduced extremely to, for instance, 0.55, 0.024 and 8.times.10.sup.-5 mW.multidot.m.sup.-2 .multidot.nm.sup.-1 for respective wavelengths of 300, 295 and 290 nm. In other words, for 295 nm it is only 1/23 of its value for 300 nm. and for 290 nm it is only about 1/700,000. For this reason, the irradiance of sunlight for 290 nm is practically thought to be zero. That is, in nature there are no ultraviolet rays having wavelengths less than 295 nm, and organic materials on the glove will be greatly affected by ultraviolet rays in such a wavelength range. For this reason, when a ultraviolet fluorescent lamp for the accelerated artificial exposure testing on polymer material radiates ultraviolet rays in a wavelength range of 290 to 295 nm, these ultraviolet rays will cause abnormal degradation of the polymer material. This leads to serious errors in the result of the accelerated artificial exposure test.
It is said that the spectral irradiance characteristic of an ultraviolet fluorescent lamp for the accelerated artificial exposure test can be controlled through control of the spectral transmittance of the glass constituting the lamp bulb. This technique is disclosed in Japanese Patent Disclosure No. 60-15,544. More specifically, the disclosure discloses an accelerated artificial exposure test apparatus which uses a bulb consisting of glass, with which the cut-on wavelength of transmittance is 295 to 300 nm. However, no glass having such characteristic has yet been developed.
FIG. 2 shows the transmittance and absorbance of glasses used in two different fluorescent lamps used for other purposes. Curve C represents the transmittance of an ultraviolet-ray-transmitting glass, which is used as bulb material of a sun lamp (i.e., FL20S.multidot.E lamp using (Ca, Zn).sub.3 (PO.sub.4).sub.2 : Tl as phosphor) and has a cut-on transmittance of about 1% for a wavelength of 275 nm. Curve D shows the transmittance of a normal soda lime glass, which is used as bulb material of a blacklight lamp (using lead-activated barium silicate as phosphor) as one kind of ultraviolet fluorescent lamp. In the ordinate of the graph of FIG. 2, the transmittance is shown on a logarithmic scale. As is seen from curve C, with the ultraviolet-ray-transmitting glass the absorbance is not sharply increased with reducing wavelength in the neighborhood of a wavelength of 300 nm, so that radiant energy in a wavelength range of 290 to 300 nm can not be sufficiently attenuated. The normal glass, on the other hand, shows high absorbance in the neighborhood of 300 nm, as seen from curve D. However, the extent of decrease of absorbance with increasing wavelength (i.e., the slope of the curve) is extremely gentle compared to the case of sunlight.
As has been shown, although the absorbance of glass is controllable between curves C and D, the slope of curve can not be varied. For this reason, glass which well transmits ultraviolet rays with a wavelength of 300 nm also well transmits ultraviolet rays with wavelengths of 290 to 295 nm, while glass well absorbing ultraviolet rays of 290 to 300 nm also well absorbs ultraviolet rays of 290 to 295 nm. Therefore, it is difficult to simulate the spectral irradiance characteristic of sunlight with that of an ultraviolet fluorescent lamp by controlling the sole spectral transmittance characteristic of the glass constituting the lamp bulb.